Tuesday, August 16, 2016

"The problem is that the current standard audio specifications for headphones and loudspeakers are almost useless in terms of indicating how good or bad they sound." —Sean OliveRead more at S&V Magazine

In May 2016, I was interviewed by editor Bob Ankosko in Sound&Vision Magazine about my views of where audio currently is,and where it is going. You can read the interview here. One of the recurring questions that I get asked is whether people really care about sound quality anymore. The fact that a recent study found 55% of Americans typically listen to music through their laptop speakers doesn't bode well for the immediate future. While the recent focus has been on the poor quality of the source material (e.g. compressed MP3), a typical laptop speaker system won't produce the bottom 3-4 octaves of music whether or not the music is compressed or recorded in high resolution (e.g. 24-bit, 96 kHz).

In terms of home loudspeakers, the trend is smaller size, fewer number of loudspeakers, and wireless. Sound Bars and small, powered wireless speakers are what consumers currently want in their homes. The current challenge for engineering is to build high quality systems with these features but still deliver good sound for prices that consumes will pay. The fact that more consumers are expecting a high quality (and branded) audio system in their automobiles suggests that the desire to have good audio is not dead.

Friday, April 22, 2016

Fig. 1 The Harman Headphone Virtualizer App allows listeners to make double-blind comparisons of different headphones through a high-quality replicator headphone. The app has two listening modes: a sighted mode (shown) and a blind mode (not shown) where listeners are not biased by non-auditory factors (brand, price, celebrity endorsement,etc). Clicking on the picture will show a larger version.

Early on in our headphone research we realized there was a need to develop a listening test method that allowed us to conduct more controlled double-blind listening tests on different headphones. This was necessary in order to remove tactile cues (headphone weight and clamping force), visual and psychological biases (e.g. headphone brand, price, celebrity endorsement,etc ) from listeners' sound quality judgements of headphones. While these factors (apart from clamping force) don't physically affect the sound of headphones, our previous research [1] into blind vs. sighted listening tests revealed their cognitive influence affects listeners' loudspeaker preferences [1], often in adverse ways. In sighted tests, listeners were also less sensitive and discriminating compared to blind conditions when judging different loudspeakers including their interaction with different music selections and loudspeaker positions in the room. For that reason, consumers should be dubious of loudspeaker and headphone reviews that are based solely on sighted listening.

While blind loudspeakers listening tests are possible through the addition of an acoustically-transparent- visually-opaque-curtain, there is no simple way to hide the identity of a headphone when the listener is wearing it. In our first headphone listening tests, the experimenter positionally substituted the different headphones onto the listener's head from behind so that the headphone could not be visually identified. However, after a couple of trials, listeners began to identify certain headphones simply by their weight and clamping force. One of the easiest headphones for listeners to identify was the Audeze LCD-2, which was considerably heavier (522 grams) and more uncomfortable than the other headphones. The test was essentially no longer blind.

Headphone virtualization is done by measuring the frequency response of the different headphones at the DRP (eardrum reference point) using a G.R.A.S. 45 AG, and then equalizing the replicator headphone to match the measured responses of the real headphones. In this way, listeners can make instantaneous A/B comparisons between any number of virtualized headphones through the same headphone without the visual and tactile clues biasing their judgment. More details about the method are in the slides and AES preprint.

An important questions is: "How accurate are the virtual headphones compared to the actual headphones"? In terms of their linear acoustic performance they are quite similar. Fig. 2 compares the measured frequency response of the actual versus virtualized headphones. The agreement is quite good up to 8-10 kHz above which we didn't aggressively equalize the headphones because of measurement errors and large variations related to headphone positioning both on the coupler and the listeners' head.

Fig. 2 Frequency response measurements of the6 actual versus virtualized headphones made on a GRAS 45 AG coupler with pinna. The dotted curves are based on the physical headphone and the solid curves are from the virtual (replicator) headphone. The measurements of the right channel of the headphone (red curves) have been offset by 10 dB from the left channels (blue curve) for visual clarify. Clicking on the picture will show a larger version.

More importantly, "Do the actual and virtual headphones sound similar"? To answer this question we performed a validation experiment where listeners evaluated 6 different headphone using both standard and virtual listening methods Listeners gave both preference and spectral balance ratings in both standard and virtual tests. For headphone preference ratings the correlation between standard and virtual test results was r = 0.85. A correlation of 1 would be perfect but 85% agreement is not bad, and hopefully more accurate than headphone ratings based on sighted evaluations.

The differences between virtual and standard test results we believe are in part due to nuisance variables that were not perfectly controlled across the two test methods. A significant nuisance variable would likely be headphone leakage that would affect the amount of bass heard depending on the fit of the headphone on the individual listener. This would have affected the results in the standard test but not the virtual one where we used an open-back headphone that largely eliminates leakage variations across listeners. Headphone weight and tactile cues were present in the standard test but not the virtual test, and this could in part explain the differences in results. If these two variables could be better controlled even higher accuracy can be achieved in virtual headphone listening.

Fig.3 The mean listener preference ratings and 95% confidence intervals shown for the headphones rated using the Standard and Virtual Listening Test Methods. The Standard Method listeners evaluated the actual headphones with tactile/weigh biases and any leakage effects. In the Virtual Tests, there were no visual or tactile cues about the headphones. Note: Clicking on the picture will show a larger version.

Some additional benefits from virtual headphone testing were discovered besides eliminating sighted and psychological biases: the listening tests are faster, more efficient and more sensitive. When listeners can quickly switch and compare all of the headphones in a single trial, auditory memory is less of a factor, and they are better able to discriminate among the choices. Since this paper was written in 2013, we've improved the accuracy of the virtualization in part by developing a custom pinnae for our GRAS 45 CA that better simulates the leakage effects of headphones measured on real human subjects [3].

Finally, it's important to acknowledge what the virtual headphone method doesn't capture: 1) non-minimum phase effects (mostly occurring at higher frequencies) and 2) non-linear distortions that are level-dependent. The effect of these two variables on virtual headphone test method have been recently tested experimentally and will be the topic of a future blog posting. Stay tuned.

The 1-hour lecture gave an overview of what are the current best practices in designing a modern-day loudspeaker.

The proof of good loudspeaker design is ultimately judged on how good it sounds. Dr. Sean Olive (me), Acoustic Research Fellow at Harman International presented an overview of the science of evaluating loudspeakers, which included test results from a competitive benchmarking of the new Revel Concerta 2 M16 (designed by Mark Glazer) against three competitors. The results of the listening test results were generally predictable based on the set of anechoic measurements made of the different loudspeakers.

Following the lecture, we got a tour of the University's engineering facilities, which include some impressive 3D laser scanning tools for studying the vibrational modes of loudspeakers. We heard some very novel flat-panel loudspeakers with vibrational mode control developed by the Ph.D students and Professor Bocko, followed by presentations of research projects undertaken by the Masters and Ph.D. engineering students who are working in acoustics and audio-related research. Overall, the quality of acoustic and music research being done there is impressive. As always, Professor Bocko was a gracious host, and we look forward to a return visit (hopefully in the summer or fall months).

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About Me

Sean Olive is Acoustic Research Fellow for Harman International, a major manufacturer of audio products for consumer, professional and automotive spaces. He directs the Corporate R&D group, and oversees the subjective evaluation of new audio products including Harman's OEM automotive audio systems. Prior to 1993, he was a research scientist at the National Research Council of Canada where his research focused on the perception and measurement of loudspeakers, listening rooms, and microphones. Sean received a Bachelors degree in Music from the University of Toronto, and his Masters and Ph.D. degrees in Sound Recording from McGill University in Montreal. His Ph.D. research was on room acoustic adaptation and the acoustical interaction between loudspeakers and rooms. Dr. Olive has written over 30 research papers on the perception and measurement of audio for which he was awarded the Audio Engineering Society (AES) Fellowship Award in 1996, and two Publication Awards (1990 and 1995). Sean is the current President of the Audio Engineering Society. For more info see www.linkedin.com/in/seanolive